A fuel cell converts chemical energy directly into electrical energy. Most fuel cells comprise a cathode or air electrode 1 and an anode or fuel electrode 3, separated by an electrolyte 5 (FIG. 1). At the air electrode 1, oxygen is ionized and the oxide ions migrate through the electrolyte to the fuel electrode 3. At the fuel electrode 3, hydrogen is ionized and the hydrogen ions react with the oxide ions to form water and release electrons. The released electrons then travel from the fuel electrode 3 to the air electrode 1 through a load-containing connection, thereby completing the circuit and providing a small amount of direct electrical current. It is well known in the art that ion quantities can vary, additional or other ion constituents can be used, and ion and electron directions can be reversed.
A fuel cell based power generation system typically comprises a plurality of electrically interconnected fuel cells. The system usually uses a hydrogen-bearing fuel (e.g. natural gas, methane, hydrogen) at the anode, and an oxidant (e.g. air, oxygen) at the cathode. A schematic arrangement of one such system, which uses solid oxide electrolyte fuel cells (SOFC), is described in U.S. Pat. No. 4,395,468.
Because fuel cells are efficient, use plentiful and renewable fuels, do not require direct combustion and produce low emissions, they are a very attractive energy source. However, although the basic electrochemical processes and schematic arrangements of fuel cell based power generation systems are well understood, engineering solutions necessary to lower fabrication costs and make such systems an economical alternative to fossil fuel and other power generation systems remain elusive.
One technical problem with conventional fuel cells involves the application of the interlayer to the air electrode. The applied interlayer should advantageously possess and maintain certain properties during a lifetime of operation under fuel cell operating conditions with various fuels, including varying temperatures (e.g. about 25–1200° C., preferably about 700–1000° C.) and pressures (e.g. about 0.5–5 atm, preferably about 1–3 atm). These properties include: high electrical conductivity, large electrochemically active interface area, oxidant permeability, the ability to inhibit degradation of the air electrode by halide vapors, the ability to inhibit long term metal diffusion from the air electrode to the electrolyte, the ability to at least partially infiltrate into the underlying air electrode substrate, strong adherence to the underlying substrate and interconnect, good chemical and physical stability, thermal cyclability, good ion transfer, and low fabrication costs.
One popular type of interlayer composition is a ceria-containing interlayer such as those described in U.S. Pat. Nos. 5,106,706, 5,516,597, and 6,139,985. A successful process used to apply such an interlayer onto an underlying air electrode substrate involves applying a liquid or slurry interlayer material onto the air electrode, followed by a drying step to remove excess liquid or slurry, and then a sintering cycle to densify the interlayer, such as those described in U.S. Pat. Nos. 4,547,437, 4,598,467, 5,106,706 and 5,516,597. This process produces an interlayer that can generally meet the above-described technical properties, but which is quite expensive and time-consuming to manufacture. For example, such a process requires a costly and time consuming three-step application process.
Another popular method of applying an interlayer onto an air electrode involves electro-chemical vapor deposition (EVD) of gaseous reactants, such as those described in U.S. Pat. Nos. 4,597,170, 4,609,562, 5,993,989 and 6,139,985. However, like with the liquid infiltration and slurry coating techniques, although the EVD process can be used to successfully produce an interlayer that at least partially infiltrates into the air electrode and generally meets above-described technical properties, it is quite expensive and time-consuming.
Other attempts to reduce interlayer fabrication costs include plasma spraying (e.g. air plasma spraying “APS”, vacuum plasma spraying “VPS”, plasma arc spraying, flame spraying) which generally involves spraying a molten powdered metal or metal oxide onto an underlying substrate surface using a plasma thermal spray gun to form a deposited layer having a microstructure generally characterized by accumulated molten particle splats. Plasma spraying techniques are described in U.S. Pat. Nos. 3,220,068, 3,839,618, 4,049,841, and U.S. Pat. Nos. 3,823,302 and 4,609,562 generally teach plasma spray guns and use thereof. Although plasma spraying has been used for fabrication of certain fuel cell layers, such as those described in U.S. Pat. Nos. 5,085,742, 5,085,742, 5,234,722 5,527,633 (plasma sprayed electrolyte) U.S. Pat. No. 5,426,003 (plasma sprayed interconnect), U.S. Pat. No. 5,516,597 (plasma sprayed interlayer) U.S. Pat. No. 5,716,422 (plasma sprayed air electrode) and Invention Registration No. H1260 (plasma sprayed air electrode, electrolyte and fuel electrode), use of such plasma spraying techniques have been of limited value when used to apply an interlayer onto an underlying substrate because they cause the interlayer to infiltrate into the air electrode rather than remaining on the air electrode surface. Moreover, these conventional plasma spraying techniques make it extremely difficult, if not impossible, to apply the interlayer as a thin uniform layer, which is important for efficient fuel cell resistance and other reasons.
Another shortcoming of known plasma sprayed interlayers involves the subsequent application of the electrolyte onto the interlayer. In particular, if an interlayer is plasma sprayed onto the air electrode, then, if the electrolyte is subsequently plasma sprayed onto the interlayer, this second plasma spraying acts as a kind of grit blaster that removes certain portions of the plasma sprayed interlayer. This phenomenon is especially evident with ceria-containing interlayers, as the ceria particles tend to be easily removed by the subsequent plasma spraying of the electrolyte.
Thus, fuel cell fabricators heretofore have been left to choose between one of two unsatisfactory interlayer application methods: either (1) apply the interlayer via a costly infiltration or ECVD technique and then apply the electrolyte via a low cost plasma spraying technique, or (2) apply the interlayer via a low cost plasma spraying technique and then apply the electrolyte via a costly ECVD or infiltration technique.
There is thus a need for an interlayer and a method for making the interlayer that can generally achieve above-described technical properties and can be applied onto an underlying air electrode at a low cost.